This invention relates to a method in which a solution containing a number of heavy metal elements, such as, actinides, e.g., uranium, thorium, transuranium elements, and the like, as well as lead, cadmium, chromium, mercury, iron, and the like, are contacted with a tannin adsorbent to adsorb the heavy metal elements onto the adsorbent, and separating the heavy metal elements from the solution.
Also, it relates to a method for eluting the heavy metal elements from the adsorbent having the heavy metal elements adsorbed therein so as to regenerate the adsorbent.
Unexamined Published Japanese Patent Application No. 3-206094 (U.S. patent application Ser. No. 07/631,946, now U.S. Pat. Nos. 5,158,711) of the present inventors, discloses a method for adsorbing a heavy metal element onto an adsorbent by contacting a solution containing a heavy metal element with a tannin adsorbent. In this method, tannin is dissolved in an aldehyde aqueous solution; ammonia is added to the resulting solution to form a precipitate; and the precipitate is aged to obtain an insoluble tannin which can adsorb nuclear fuel elements or iron ions.
The inventors have also disclosed a method for regenerating a tannin adsorbent having adsorbed actinides therein by contacting the adsorbent with a dilute mineral acid to elute the actinides therefrom when the adsorbing ability of the tannin adsorbent is lowered (Unexamined Published Japanese Patent Application No. 3-293597).
However, a problem occurs in the former method when a waste liquid containing a number of heavy metal elements is treated, because an adsorbent capable of adsorbing all of the heavy metal elements has not been known. It has thus been impossible to efficiently separate all the heavy metal elements from the waste liquid with a single adsorbant.
A problem with the latter method is that when the adsorbent is regenerated by using the dilute mineral acid, the extent of regeneration in the adsorbent varies since the elution rates of the adsorbed actinides are different each other.